Superconducting quantum interference devices, known as SQUIDs, have been developed and applied to many problems in physics, earth science and biology. These are highly sensitive static detectors of magnetic flux which can be built to have intrinsic energy sensitivities approaching the quantum limit. Examples of SQUID devices being used as gradiometers, magnetometers, and galvanometers is found in an article by M. B. Ketchen et al, J. Appl. Phys. 49, 7, page 4111, July 1978 and in J. Clarke, IEEE Trans. on Electron Devices, Vol. ED-27, p. 1896 (1980) and J. Clarke, "Superconducting Applications: Squids and Machines", edited by B. B. Schwartz and F. Foner, Plenum Press, N.Y. 1977.
In the gradiometer of Ketchen et al, two pick-up loops are used together with a SQUID, the loops and the SQUID being made of thin films deposited on a planar substrate. Typically an input coil is used to couple the signal from the pick-up loop to the SQUID to allow shielding of the SQUID functions from the external magnetic field. Both RF and dc SQUIDs have been applied to magnetometry. The RF SQUID is equivalent to a superconducting ring having a single weak link or Josephson tunnel device coupled to a resonant circuit driven by a constant current source at a selected RF frequency. Both the Q-factor and the resonant frequency of the circuit are modified by the coupling to the SQUID depending on the magnetic flux through the ring. On the other hand, a dc SQUID is one in which a superconducting loop incorporates multiple junctions, for example, two Josephson junctions, in parallel. For this type of SQUID, the maximum supercurrent across the device, the critical current, is a periodic function of the magnetic flux enclosed in the loop. Dc SQUIDs are usually operated in a resistive mode at constant current in which the total current is due in part to superconducting electrons and in part to normal electrons. A voltage signal is then picked off a convenient operating point of the corresponding current-voltage curve. Changes in this voltage are a function of changes in the magnetic flux contained within the loop.
In recent years, a number of workers have reported planar dc SQUIDs with improved intrinsic energy sensitivities, ultimately approaching the quantum limit. For example, reference is made to the following articles:
1. E. L. Hu et al, IEEE Trans. Magn., Mag-15, 585 (1974) PA0 2. M. B. Ketchen et al, Appl. Phys. Lett., 35, 812 (1979) PA0 3. R. F. Voss et al, Appl. Phys. Lett., 37, 656 (1980) PA0 4. M. W. Cromar et al, Appl. Phys. Lett., 38, 723 (1981) PA0 5. D. J. vanHarlingen et al, Appl. Phys. Lett., 41, 197 (1982) PA0 1. M. B. Ketchen et al, Appl. Phys. Lett., 40, 736 (1982) PA0 2. D. J. deWall et al, Appl. Fhys. Lett., 42, (1983) PA0 3. B. Muhlfelder et al, IEEE Trans. Magn., MAG-19, 303 (1983) PA0 1. D. S. McLachlan et al, Rev. Sci. Inst., 39, 1340 (1968) PA0 2. A. K. Drukier et al, Lettre al Nuovo Cimento, 14, 300 (1975)
Planar coupling schemes have also been introduced to provide more efficient coupling of high resolution SQUIDs to input circuits having useful inductances of the order of 1 .mu.H. Representative examples of these planar coupling schemes include the following references:
Applications envisioned for this new generation of SQUIDs have tended to involve making more sensitive instruments of the variety already in use now for a number of years. That is, dc SQUIDs having small size and high sensitivity are made, and can be substituted into instruments such as the susceptometer manufactured by S. H. E. Corporation of San Diego, Calif. These commercial susceptometers are used to measure samples having typical dimensions of approximately 1 cm. Sensitivites on the order of 10.sup.-8 emu over a temperature range of 4.2.degree. K. to 400.degree. K. can be achieved with magnetic fields of up to 50 kG.
Such commercial susceptometers consist of a roughly balanced wire wound gradiometer (no low temperature balance adjustment) connected to an approximately 2 .mu.H input coil of a conventional RF SQUID. Static ambient fields of up to approximately 50 kG are applied with a superconducting solenoid magnet. The magnetic field is turned on with the gradiometer coils driven normal by a heater. With the field at the desired value, the heater is turned off, trapping some flux but no current in the gradiometer pick-up loops/input coil circiut. The RF SQUID, in the meantime, is highly shielded from the applied magnetic field. The field coil gradiometer pick-up loops are all within a large superconducting shield for electrical isolation. The sample to be measured is periodically (every few seconds to a minute) moved into and out of one of the gradiometer pick-up loops. The amplitude of the signal registered by the SQUID is proportional to the susceptibility of the sample. The sample chamber is thermally isolated from the cryogenic environment so that the sample temperature can be varied over a wide range. The roughly balanced gradiometer pick-up coil configuration helps with noise rejection, although in a more highly shielded environment a straight magnetometer arrangement would work equally well.
In many applications in physics, it is desirable to be able to look at the magnetic properties of small particle sizes in fields of 0-30 G. Such instruments require extremely small junctions in the dc SQUID in order to have enhanced sensitivity of the SQUID, and extremely small pick-up loops to allow good coupling to the sample. Further, the fluctuation of certain parameters, such as temperature, scales inversely with volume. The ability to study small samples makes possible the measurement of fluctuation and noise effects that are averaged out and undetectable in larger samples.
To precisely examine the physical properties of certain materials very small samples are required. If large area samples are used, essential features can become averaged and the measured values are not truly representative of the actual values. In the case of thin film samples, these are particularly hard to measure because of the geometry effects that occur to cloud the meaning of the data that is obtained. Of course, it is also difficult to make a truly homogeneous film over large dimensions, and for this additional reason it is desirable to be able to precisely examine very small particles and films.
Another area in which accurate investigation of small particles and films is required is where larger samples are unavailable. For example, single crystals of materials that exhibit both superconductivity and ferromagnetism cannot be made in large samples. For these materials, it is necessary to look at a single crystal to investigate the material in order to separate out what behavior is truly intrinsic to an individual crystal as opposed to behavior resulting from interactions at grain boundaries. The instrument described here is ideally suited for the study of such samples including an investigation of crystal size efforts that may become evident as dimensions are decreased to on the order of small characteristic lengths characterizing internal interactions within the material.
Accordingly, it is an object of this invention to provide a SQUID susceptometer which is miniature and integrated on a single chip, and which is ideally suited for the study of magnetic properties of small particles and thin film samples at cryogenic temperatures.
It is another object of the present invention to provide a SQUID susceptometer which has increased sensitivity over presently available small particle susceptometers.
The use of small wire-wound coils and conventional electronics to measure the susceptibility changes of small particles going through their superconducting transition is described in the following references, both of which describe instruments that are several orders of magnitude less sensitive than the present instrument. They are:
It is another object of the present invention to provide a novel miniature SQUID susceptometer which is a broad band instrument that will allow the measurement of magnetic noise spectra as well as the measurement of temperature dependent susceptibilities.
It is another object of the present invention to provide a miniature SQUID susceptometer capable of measuring extremely small samples and in which inductance values are significantly reduced over those encountered in commercial SQUID susceptometers.
It is another object of the present invention to provide a miniature SQUID susceptometer having extremely small sizes integrated in a planar fashion on a single chip, and in which the dimensions of both the junctions of the SQUID and the coils in the instrument are limited only by the lithography used to produce the instrument.
It is another object of the present invention to provide a miniature SQUID susceptometer in which the scale for the size of the sample being investigated is set by the same lithography that limits the size of the junctions in the SQUID.
It is another object of the present invention to provide a miniature SQUID susceptometer integrated on a single chip in which the dimensions of all components of the SQUID instrument are very small and in which all components are fabricated by planar lithography techniques on a single chip, and wherein the same lithography techniques can be used to deposit the sample to be measured by the instrument.